I. Field
The present invention relates generally to communication, and more specifically to data transmission in a wireless multiple-access communication system.
II. Background
A wireless multiple-access system can concurrently support communication for multiple wireless terminals on the forward and reverse links. The forward link (or downlink) refers to the communication link from the base stations to the terminals, and the reverse link (or uplink) refers to the communication link from the terminals to the base stations. Multiple terminals may simultaneously transmit data on the reverse link and/or receive data on the forward link. This may be achieved by multiplexing the data transmissions on each link to be orthogonal to one another in time, frequency, and/or code domain. The orthogonality ensures that the data transmission for each terminal does not interfere with the data transmissions for other terminals.
A multiple-access system typically has many cells, where the term “cell” can refer to a base station and/or its coverage area depending on the context in which the term is used. The coverage area of a base station may also be partitioned into multiple sectors. Data transmissions for terminals in the same cell may be sent using orthogonal multiplexing to avoid “intra-cell” interference. However, data transmissions for terminals in different cells may not be orthogonalized, in which case each terminal would observe “inter-cell” interference from other cells. The inter-cell interference may significantly degrade performance for certain disadvantaged terminals observing high levels of interference.
To combat inter-cell interference, a multiple-access system may employ a frequency reuse scheme whereby not all frequency bands or subbands available in the system are used in each cell. As an example, cells that are adjacent to each other may use different frequency bands, and the same frequency bands may be reused only by non-adjacent cells. The inter-cell interference observed in each cell with frequency reuse is reduced relative to the case in which all cells use the same frequency bands.
The operating environment for a wireless terminal may change over time due to various factors such as, e.g., movement by the terminal across cells, fading, multipath, interference effects, and so on. Handoff is a useful technique for maintaining good channel conditions for terminals on the edge of cell coverage. Some conventional systems, such as a Time Division Multiple Access (TDMA) system, support “hard” handoff whereby a terminal first breaks away from a current serving base station and then switches to a new serving base station. Hard handoff can provide switched-cell diversity against path-loss and shadowing at the cost of a brief interruption in data transmission. A Code Division Multiple Access (CDMA) system supports “soft” and “softer” handoffs, which allow a terminal to simultaneously communicate with multiple cells (for soft handoff) or multiple sectors (for softer handoff). Soft and softer handoffs can provide additional mitigation against fast fading.
Implementation of soft and softer handoffs is complicated by frequency reuse because not all frequency bands/subbands are available for use in each cell. There is therefore a need in the art for techniques to support soft handoff with frequency reuse.